Eukaryotic protein-encoding genes are regulated by gene-specific regulatory factors that direct the spatial and temporal programs of gene expression. To understand how regulatory factors control RNA polymerase 11 (pol 11) activity, the components required for gene activation by regulatory factors have been purified. Partially purified natural TFIID mediates gene activation by regulatory proteins, and appears to have a molecular weight of several 100 kDa. In contrast, cloned TFIID is much smaller (38 kDa), and fails to mediate activation by trans-acting factors. Given that both forms mediate basal levels of transcription, it appears that natural TFIID contains associate component(s) that are selectively required for a regulatory factor function. Both human and Drosophila native TFIIDs have been isolated using an antibody-affinity method. The promoter region of the human gfa gene, encoding glial fibrillary acidic protein (GFAP) has been found to contain two elements that can independently specify the transcription startpoint. One of the elements is a TATA box found 25 bp upstream from the transcription startpoint, while the other is located between 10 and 50 bp downstream from the transcription initiation site. A downstream initiation element affects the TATA box-binding as well as the in vitro transcription activity of TFIID.